In the past, a light protection mask has used a light blocking plate to protect a human operator's eyes from an arc light that is generated during a welding operation. More recently, a welding-operation, liquid crystal, protection mask has been developed, which mask is equipped with a liquid crystal plate that is capable of blocking a light by instantaneously reducing the light transmittance of a liquid crystal. These known welding-operation, liquid crystal, protection masks, however, have been found to have some drawbacks. When many human operators are conducting welding operations at the same time, the light transmittance of the light blocking plate being used by one human operator may be undesirably actuated by nearby arc lights of other welders and also by illumination lights.
Because known welding-operation, liquid crystal, protection masks are designed for the purpose of sufficiently protecting the eyes of a human operator, they are usually constructed so that light transmittance during a light blocking operation is extremely small. As a result, a human operator typically can recognize nothing but a welding point when the device is activated. For this reason, when a mistaken light blocking operation occurs that is out of control of a human operator during the welding operation, the operator will suddenly become unable to see, creating an extremely dangerous condition for them.
In order to prevent this situation from occurring, it has been suggested that a light blocking cylindrical member be used in front of a light receiving member, which cylindrical member consists of an ultraviolet light detecting element, so as to form an improved welding-operation, liquid crystal, protection mask that only responds to incident light that arrives from the front (see Japanese Unexamined Patent Application Publication No. 2-159272). This known welding-operation, liquid crystal, protection mask is beneficial in that it can treat an arc light entering at an incidence angle of a certain narrow range from a position ahead of the light receiving member.
The arc light detector, however, not only responds to an arc light entering at an incidence angle of a narrow range from a position ahead of the light receiving member, but also responds to other lights that reach the light receiving member from other directions, including arc lights generated and reflected during the welding operations of other human operators. As a result, the cylindrical light blocking member used in the above-discussed liquid crystal protection mask fails to effectively avoid a problem of detecting other lights. Thus, it has been proved difficult to obtain a desired and satisfactory detection precision.